Ammonia (NH3) can be generated in various conditions or it is used as a reactant in some processes. Ammonia that has escaped into the air or the surface waters is harmful to humans and the environment; therefore, it is not allowed to escape in considerable amounts along with effluent gases from the processes. The authorities have set the highest allowable contents or total emission amounts for ammonia emissions. Ammonia is also used for the adjustment of pH in solutions. The effluent gases of processes may comprise small or large amounts of ammonia. Urea- or NH3-SCR is a common removal method of nitrogen oxides, which can release small amounts of ammonia. Ammonia is also generated from nitrogen compounds in low-oxygen conditions, such as stoichiometric or rich combustion mixtures, where the fuels are solid, liquid or gaseous. Ammonia can be generated in fuel refining. Ammonia can be generated, for example, when purifying soil, other solid materials or liquids that comprise ammonia as a chemical or derivatives of ammonia, or it can be generated naturally in low-oxygen conditions. In agriculture, large amounts of ammonia emissions are generated. When gasifying fuels, ammonia can also be generated together with HCN from the nitrogen contained in the fuel.
For the removal of ammonia from gases or liquids, catalytic decomposition, adsorption, absorption or thermal decomposition methods have been used. In reductive conditions, NH3 decomposes or reacts thermally or catalytically, forming mainly nitrogen (N2) and small amounts of laughing gas (N2O). In mixtures comprising excess amounts of oxygen, NH3 can form nitrogen oxides (NO, NO2, N2O) in the thermal or catalytic methods, and the selectivity to nitrogen is a critical factor. The formation of laughing gas is also higher than in reductive or stoichiometric conditions.
In the catalytic decomposition of ammonia, two different operating conditions can definitely be distinguished, i.e., decomposition in a rich mixture (low amounts of oxygen) or a lean mixture (excessive amounts of oxygen). When there are excessive amounts of oxygen, the main reaction is often that of NH3 into nitrogen oxides, and the selectivity to nitrogen is a problem. When there are low amounts of oxygen, nitrogen oxides are not easily generated but the oxygen required must be obtained selectively before carbon monoxide, hydrocarbons or hydrogen, and the oxygen supply is limited. The decomposition of NH3 at high temperatures and pressures is thermodynamically limited. Ammonia has been decomposed by alumina-based Ni, Ru catalysts (Mojtahedi et al., Fuel Proc. Tech. 45 (1995) 221). For the removal of nitrogen compounds from gasifying gases, a mixture formed by the oxide of the metals (such as Fe oxide) of the fourth cycle of the group VIII of the periodic system and alkali metal or alkali earth metal carbonate or oxide (such as CaO) (FI 904697, Leppälahti and Simell) have been used. Generally, Mo, W, Re, Fe, Co, Rh, Ni, Pt, Cu and V (Catal. Sci. Tech.1 (1981) 118, or U.S. Pat. No. 5,055,282) have been mentioned to be the chemical elements that are suitable to the decomposition of NH3 in the catalyst. It has been observed that Ru/alumina decomposes NH3 at a temperature of as low as 500° C. (U.S. Pat. No. 5,055,282). The Ru/alumina catalyst may also have included various alkali metals and alkali earth metals (K, Li, Na, Cs, Ca, Ba, Mg). The decomposition of ammonia has also been exploited in the industrial processing of steel products.
Selective catalytic oxidation (SCO) refers to a method, wherein NH3 is selectively oxidized into nitrogen (N2) with oxygen. This method relates to a phased air supply in combustion or by gasifying, wherein the oxidation of NH3 into NOx can be prevented. There are difficulties in preventing the formation of NOx and the integration of the method into the combustion/gasifying process in question. The catalysts used included MoO3, V2O5, Bi2O3, and PbO, MoO3/SiO2 catalysts (Boer et al. Stud. Surf. Sci. Catal. 72 (1992) 133). The poor activity, selectivity or durability of the catalysts are typical problems, especially, when the operating temperatures are normally high (>400° C.).